Underwater Vehicle Communication Using Electromagnetic Fields in Shallow Seas

Some unavoidable shortcomings of acoustic wave based communications leads to the use of low frequency electromagnetic (EM) waves for underwater vehicle communication. Electric conductivity, causing frequency dependent attenuation, limits communication distance and data transfer rate in sea environment. An enhancement in range and bandwidth can be achieved by: 1) allowing EM signals to cross seawater-toair boundary and achieve long-range horizontal communication using air path, followed by air to water signal transmission, if needed, and 2) exploring guided waves phenomenon at the water side of the seawater-air interface. A computational investigation to compare the EM wave propagation characteristics in seawater, in both sides of the seawater-air interface within a thin layer, and in air after crossing interface, in order to determine the optimum operating frequencies, ranges of communication, antenna orientation, and vertical offset of submerged antenna is presented in this paper. It is shown that with approximately 70 dB loss of signal strength, guided electric fields through air- water interface can be transmitted to a distance of 1 km at 10 KHz when the transmitter is 5 m below the interface.